Through the years, a great number of devices have been assembled to measure air pressure with varying degrees of success and accuracy. For example, simple mercury barometers can be assembled by providing a glass tube having one end sealed and being held vertically with the open end submerged in a bottle or dish such that the difference in height of mercury in the jar and the tube measures the air pressure. Simple barometric devices can also be provided by inverting a bottle which has been partly filled with water such that the neck of the bottle is under the surface of more water placed in a dish or saucer.
Such an arrangement is also used in various forms of poultry and cattle feeder devices. This arrangement is less accurate than the mercury arrangement as a result of, among other reasons, inherent evaporation of the water into the ambient atmosphere. Additionally, commercially available barometer devices commonly feature relatively low sensitivity to pressure changes, and, consequently, do not register small changes and do not demonstrate larger changes in a dramatic or interesting manner.
Another pressure measuring apparatus is shown and described in U.S. Pat. No. 2,439,342, which issued to W. J. Hudson on Apr. 6, 1948. The Hudson patent discloses the use of a movable hollow member which, immersed in liquid, assumes an equilibrium position of buoyant suspension corresponding to the ambient pressure surrounding the apparatus. Particularly, the Hudson apparatus includes an external container housing a body of liquid having a very low vapor pressure. Within the liquid is an inverted hollow member having an open bottom, and a space occupying equilibrium restoring body which is rigidly fixed at its lower end to a guiding frame within the container. The hollow member is supported for vertical movement within the guiding frame which engages the internal walls of the container to provide a positive support for the space occupying body over which the hollow member is inverted.
When there is a pressure increase imposed on the liquid within the Hudson container, the volume of air trapped within the hollow member is compressed and the hollow member moves downwardly. A new position of equilibrium is achieved when the increase in exposure of the equilibrium restoring body equals the decrease in the volume of the trapped gas. Hudson teaches that the sensitivity of this device can be selected by choosing the ratio of the cross-sectional area of the hollow member to that of the equilibrium restoring body, suggesting that when the ratio is large, the sensitivity is high.
The Hudson device further includes a correction for variations in the performance of the device due to temperature differences. Particularly, a series of reference marks is to be placed on the stationary equilibrium restoring body, and the particular reference mark used in monitoring pressure differences is chosen depending upon the prevailing temperature. In a second embodiment, Hudson contemplates the use of an integral thermometer placed within the equilibrium restoring body such that the upper surface of the liquid in the thermometer forms the reference mark against which pressure differences are to be monitored. Another embodiment includes a dynamic equilibrium restoring body which is carried upon a bi-metallic mounting arm designed to react proportionally to temperature. Allegedly this bi-metallic mounting arrangement automatically adjusts the position of the equilibrium restoring body in accordance with the prevailing temperature. As can be seen, however, the Hudson pressure measuring apparatus is relatively complex, requiring the interaction of a plurality of parts which must be carefully mounted within the external container, requiring precise sizing and complicating assembly of these parts, and making uniformity and accuracy difficult to achieve.
Another device for indicating changes in atmospheric pressure is shown in U.S. Pat. No. 2,690,675, which issued to T. R. Farrier on Oct. 5, 1954. The Farrier fishing indicator utilizes three floats which each have different specific gravities. The device is intended to indicate the relative chances of successful fishing based upon existing atmospheric pressure, and includes an adjustment mechanism to ensure that the pressure of the fluid within the container is compensated for the normal atmospheric pressure of 29.92 inches of mercury. This device is to include a transparent container filled with a low viscosity liquid, a plurality of floats of various and distinct specific gravities, and a diaphragm closing the container to the atmosphere.
The base of the Farrier container includes a threaded cam for adjusting the pressure on the diaphragm to adjust the device and to compensate for the particular altitude at which the device is being used. Each of the floats includes an interior hollow portion enclosing a predetermined amount of air and being open to the low viscosity liquid at its bottom surface. Upon increased atmospheric pressure, the diaphragm of the container is urged upwardly and the pressure within the container increases, thereby compressing the air within the individual floats and increasing their effective specific gravities causing them to sink. If atmospheric pressure is low, Farrier teaches that the fishing will be excellent; whereas if atmospheric pressure is high, fishing will be poor.
The Farrier device relies on the well-known "Cartesian diver" principle that the density of an object housing a predetermined volume of compressible fluids will increase in density in response to a rise in pressure, and will decrease in density as the pressure falls. Other examples of the use of Cartesian diver principles can be found in U.S. Pat. Nos. 2,345,243 (which issued to W. D. Eakin on Mar. 28, 1944) and 4,448,409 (which issued to T. Kaga et al. on May 15, 1984). These references both pertain to aquatic diving toys which respond to pressure changes to provide movement of the toy figure for entertainment purposes.
The Eakin aquatic doll includes a hollow portion having a movable diaphragm member with a weight attached, wherein changes in pressure tend to move the weight and change the volume of the hollow portion within the figure, resulting in predetermined movement of the figure within a volume of support liquid. The aquatic toy can undergo a sequence of predetermined movements initiated by relatively rapid and substantial pressure changes induced by a control device. The movements and actions can be predetermined and varied by adjusting the timing, duration, and amount of pressure changes induced on the support liquid by the control device.
Similarly, Kaga et al. teach a cartesian diving toy which includes a convoluted bellows structure susceptible to elongation and shortening in response to pressure changes to provide an essentially linear reaction to those pressure changes. Kaga et al. further contemplate that pressure variations and temperature variations will both be transformed into linear movement via the convoluted bellows, and tail portions of the toy fish are linked to the bellows so that such linear movement acts to move the tail to propel the toy within its support liquid.
A pressure gauge described as taking advantage of the Cartesian diver principle is set forth in U.S. Pat. No. 2,701,966, which issued to C. Brown. The Brown pressure gauge is contemplated for substitution for a conventional tire cap to provide visual indication when the pressure in the tire has dropped below a desired inflation pressure. Brown further suggests compensation for temperature variations by careful selection of materials, and by taking into account the applicable coefficients of volumetric expansion of the various elements involved. The pressure gauge can be provided with a variable float, or with a number of floats of varying specific gravity, however, Brown states that the variable float cannot be readily fully temperature compensated. The variable float of Brown is taught as including a flexible element or thread depending below the float to effectively vary the specific gravity in accordance with the height of the float.
Heretofore, there has not been available in the industry a relatively simple apparatus for measuring variations in pressure which is easy to assemble and can provide extremely sensitive monitoring of ambient pressure variations. More accurate devices have generally required relatively complex arrangements of parts having concomitant difficulties in assembling the various parts and/or whose complex nature detracts from the appearance and usefulness of the device.